Re: [PATCH for_v29 0/8] x86/sgx: Make vDSO callable from C

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, Mar 20, 2020 at 02:57:24AM +0200, Jarkko Sakkinen wrote:
> On Wed, Mar 18, 2020 at 06:11:22PM -0700, Sean Christopherson wrote:
> > Sean Christopherson (8):
> >   x86/sgx: vdso: Remove an incorrect statement the enter enclave comment
> >   x86/sgx: vdso: Make the %rsp fixup on return from handler relative
> >   x86/sgx: vdso: Make __vdso_sgx_enter_enclave() callable from C code
> >   x86/sgx: vdso: Define a typedef for __vdso_sgx_enter_enclave
> >   selftests/x86: sgx: Zero out @result before invoking vDSO sub-test
> >   selftests/x86: sgx: Pass EENTER to vDSO wrapper instead of hardcoding
> >   selftests/x86: sgx: Stop clobbering non-volatile registers
> >   selftests/x86: Add selftest to invoke __vsgx_enter_enclave() from C
> > 
> >  arch/x86/entry/vdso/vsgx_enter_enclave.S      | 72 ++-----------------
> >  arch/x86/include/uapi/asm/sgx.h               | 61 ++++++++++++++++
> >  .../selftests/x86/sgx/encl_bootstrap.S        |  6 +-
> >  tools/testing/selftests/x86/sgx/main.c        | 17 ++++-
> >  tools/testing/selftests/x86/sgx/sgx_call.S    |  1 -
> >  tools/testing/selftests/x86/sgx/sgx_call.h    |  2 +-
> >  6 files changed, 85 insertions(+), 74 deletions(-)
> > 
> > -- 
> > 2.24.1
> > 
> 
> Might be a grazy idea but better to go through this anyway.
> 
> Given the premise that we need the carry the callback anyway in all
> cases, why won't just have the callback.
> 
> Why we absolutely need the code path that fills exception info given
> that we no matter what need to have a callback route?
> 
> Would simplify considerably to have only clear flow.

Invoking the callback uses a retpoline, which is non-trivial overhead.
For runtimes that need an assembly wrapper for other reasons, and aren't
using the untrusted stack, forcing them to implement a handler would be
painful.